Bulk acoustic wave sensor

ABSTRACT

A sensor includes a substrate  1  and at least one resonator, which includes an acoustic reflector  2 , a piezoelectric layer  5 , a first and second electrode  3, 4  placed on the same side of the piezoelectric layer  5 , and a sensing layer  6 . The sensing layer  6  reacts with a chemical or biological agent by absorption, adsorption, desorption or chemical reaction. As a result the individual frequency of a resonator changes and conclusions about the agent can be drawn. Such a sensor is very sensitive to an agent being sensed, especially when used in liquids.

This invention relates to a sensor comprising a substrate and at leastone resonator, said resonator is comprised of an acoustic reflector, apiezoelectric layer, a first and a second electrode, and a sensinglayer.

The rapid and accurate detection of chemical and biological agents isvery important for many activities such as environmental pollutiontesting or military application.

Heretofore, biological/chemical sensors have been based on eithercalorimetric or gravimetric effects.

For example, U.S. Pat. No. 5,936,150 describes a chemical sensor using athin film acoustic resonator coated with a chemically sensitive sorbentcoating. The thin film acoustic resonator has electrodes separated by athin film piezoelectric layer and is supported by a multilayer resonantacoustic isolator. In order to detect a chemical, the chemical ofinterest (usually a vapor) is brought into contact with and interactswith the exposed sorbent surface coating, causing the mass and/or themechanical properties of the sorbent coating to change. The changes inthe sorbent coating will cause a change in the resonant frequency of thethin film acoustic resonator, as the coating lies directly upon theresonator and is part of the acoustic resonant path.

The above-mentioned sensor may best be applied when detecting gaseousagents. When using the sensor in liquids the liquid will attenuate theresonator. This results in a lower sensitivity and a lower Q factor ofthe device.

It is an object of the present invention to provide a sensor that ismore sensitive when used in liquids.

This is achieved by a sensor comprising a substrate and at least oneresonator, said resonator is comprised of an acoustic reflector, a firstand a second electrode, a piezoelectric layer, and a sensing layerwherein the first and the second electrode are placed on the same sideof the piezoelectric layer.

Such a resonator has the advantage that predominately shear vibrationsare excited that may not be absorbed by liquids. As a result the excitedvibrations cannot propagate in the liquid and the attenuation of thevibrations is low. This results in lower acoustic losses of theresonator. A sensor made of such a resonator has a high sensitivity whenused in liquids.

The sensor includes a resonator having a low electric impedance.

In accordance with the invention, a highly robust and sensitive sensorarray for the detection of chemical or biological agents in liquids isobtained.

A sensor in accordance with the invention works with a very highsensitivity since every material used in the sensing layers interacts ina different degree with the agent to be detected. The obtained outputsignal is rather complicated and thus sensitive to the agent to bedetected.

The invention will be explained in more detail below with reference totwo drawings. In the drawings

FIG. 1 shows a sensor array circuitry and

FIG. 2 shows the construction of a resonator in cross-section.

The sensor according to the invention comprises a substrate and at leastone resonator. Said resonator is comprised of an acoustic reflector, afirst and a second electrode, a piezoelectric layer, and a sensinglayer.

More preferred the sensor comprises a plurality of individuallyaddressable resonators. It is highly preferred that this plurality ofresonators is arranged in a rectangular row and column configurationincluding m rows and n columns. By this arrangement a sensor array isachieved. Most preferred each resonator might be addressed individually.FIG. 1 shows a circuitry of such a sensor array comprising m·nresonators R.

If the sensor comprises a plurality of resonators, each of theresonators is separated from its neighboring resonators by such adistance that the resonant energies do not overlap.

The sensor comprises a substrate 1 which may comprise a semiconductingmaterial such as Si or GaAs or an insulating material such as glass orAl₂O₃. The resonators are deposited on the substrate 1. Each resonatorcomprises an acoustic reflector 2, a first electrode 3 and a secondelectrode 4, a piezoelectric layer 5, and a sensing layer 6.

FIG. 2 shows in cross-section a resonator according to the invention.

Most preferred the acoustic reflector 2 has a layered structure andcomprises several layers having alternating high and low acousticimpedance. These layers show a thickness of ¼ of the resonancewavelength λ. The layers having low acoustic impedance may for examplecomprise an organic or inorganic aerogel, an organic or inorganicxerogel, a foam, a low density synthetic material or SiO₂. The layershaving a high acoustic impedance may for example comprise Ta₂O₅, Si₃N₄,TiO₂, ZnO, LiNbO₃, LiTaO₃, Al₂O₃, SiC, V₂O₅, Nb₂O₅, ZrO₂, La₂O₃, WO_(x)(0<x≦3), MoO_(x) (0<x≦3), ZrC, WC, MoC, ThO₂, CeO₂, Nd₂O₃, Pr₂O₃, Sm₂O₃,Gd₂O₃, ReO_(x) (0<x≦3.5), RuO₂, IrO₂, Y₂O₃, Sc₂O₃, LiGeO₂, Bi₁₂GeO₂₀,GeO₂, MgO, yttrium-aluminium-garnet (Y₃Al₅O₁₂, YAG), yttrium-iron-garnet(Y₃Fe₅O₁₂, YIG), LiGaO₂, HfO₂, AlN, a high density synthetic material, Wor C.

The number of layers in the acoustic reflector 2 is odd-numbered and thefirst and the last layer of the layered structure comprise a materialhaving low acoustic impedance. It should be understood that althoughfive such layers are shown, a greater number may be used, and in generalfive to nine layers are desirable.

The piezoelectric layer 5 that is placed on top of the acousticreflector 2 comprises most preferentially AlN. Alternatively ZnO, KNbO₃,lead zirconate titanate (PZT), lanthanum-doped lead zirconate titanate(PLZT) or the like may be used in the piezoelectric layer 4

The first and second electrode 3, 4 are deposited on top of thepiezoelectric layer 5. The electrodes 3, 4 may comprise a metal like Pt,Al, Al:Cu, Al:Si, Mo, W or an alloy as well as an additional adhesionlayer comprising Ti, Cr, or NiCr. Most preferred the two electrodes 3, 4are interdigitated electrodes.

At the outer side of the piezoelectric layer 5 ground metallizationsand/or bond-pads (not shown) are present. The ground metallization andthe bond pads may comprise Al, Al:Si, Al:Cu, Cu, Ni, Au or combinationsthereof.

A sensing layer 6 completely or only partly covers the two electrodes 3,4. The material of the sensing layer 6 is selected with respect to theagent to be detected. The detection of the chemical or biological agentsis done by absorption, adsorption, desorption or chemical reaction.

The chemical or biological agents may comprise an atom, ion, molecule,macromolecule, organelle, or a cell. The chemical or biological agentsmay also comprise a substance in a medium including but not limited to,environmental contaminants such as trichloromethane, tetrachloromethane,trichloroethane, trichloroethylene, tetrachloroethane,tetrachloroethylene, toluene, benzene, aromatic compounds andhydrocarbon pesticides. The term “medium” as used herein means anaqueous medium, a non-aqueous liquid medium, and gases. As used herein,the terms “chemical agent” or “biological agent” also include molecules,such as proteins, glycoproteins, metal salts, ions, and the like. Theterms also include neurotransmitters, hormones, growth factors,cytokines, monokines, lymphokines, nutrients, enzymes, and receptors.The terms “chemical agent” or “biological agent” also means structuredelements such as macromolecular structures, organelles and cells,including, but not limited to, cells of ectodermal, mesodermal, andendodermal origin such as stem cells, blood cells, neural cells, immunecells, and gastrointestinal cells, and also microorganisms, such asfungi, viruses, bacteria, and protozoa.

The sensing layer 6 may for example comprise chemical reactants,antibodies, fragments of antibodies capable of binding an agent,biological receptors for particular agents, enzymes, proteins,oligonucleotides, nucleic acids (such as DNA or RNA), peptides,metallo-organic materials, or small molecules such as ligands. Thesensing layer 6 may also comprise a polymeric material that responds tothe agent being present at the sensing layer 6. Additionally, thepolymeric material of sensing layer 6 can be modified by incorporatingmolecular groups that will enhance selectivity toward target agents.

In some instances it may be difficult to obtain substances that arespecific to a single chemical or biological agent, but the presence of aplurality of resonators allows one to use several materials in thesensing layers 6, one on each resonator. Each sensing layer 6 might havea different reactivity or sensitivity to the chemical or biologicalagent being sensed. By putting together the information from the groupof resonators, the presence of the suspected agent may be inferred.Using materials on the resonators that operate by absorption,adsorption, desorption or chemical reaction, the individual frequenciesmay go up or down with exposure.

Alternatively the sensor may only be comprised of two resonators. Inthis embodiment, one of the resonators is a reference resonator and thesensing layer 6 is inactive to the agent. For example, if the detectionof the agent is done by absorption, the reference resonator does notchange its mass due to absorption because of an inactive coating.Alternatively, the reference resonator comprises a material in thesensing layer 6 from which a known fixed grade of absorption is known.

A resonator comprising both electrodes 3, 4 on the same side of thepiezoelectric layer 4 has the advantage that predominately shearvibrations are excited that may not be absorbed by liquids. As a resultthe excited vibrations cannot propagate in the liquid and theattenuation of the vibrations is low. This results in lower acousticlosses of the resonator. A sensor made of such a resonator or aplurality of such resonators has a higher sensitivity.

The application of a voltage to the electrodes 3, 4 causes thepiezoelectric layer 5 to be excited into oscillations. For this reasonthe first electrode 3 and the second electrode 4 of a resonator arefurther coupled to an oscillator circuit, capable of applying a timevarying excitation voltage between the first electrodes 3 and secondelectrodes 4, so that a time-varying electric field is generated.

1. A sensor comprising a substrate and a plurality of individuallyaddressable resonators, each of the resonators comprising: an acousticreflector on the substrate, the acoustic reflector comprising aplurality of layers having alternating high and low acoustic impedances,an uppermost layer having the low impedance; a piezoelectric layerseparated from the substrate by the acoustic reflector; first and secondresonator electrodes on an upper surface of the piezoelectric layer; anda sensing layer covering a portion of an upper surface of each of thefirst and second resonator electrodes.
 2. The sensor of claim 1, whereinthe first and the second electrodes are interdigitated electrodes. 3.The sensor of claim 1, wherein the sensing layers the plurality ofresonators comprise different materials.
 4. The sensor of claim 1,wherein the plurality of layers of the acoustic reflector is an oddnumber of layers.
 5. The sensor of claim 1, wherein the portion of thefirst and second resonator electrodes covered by the sensing layercomprises a portion of an upper surface of each of the first and secondresonator electrodes.
 6. A sensor comprising: a substrate; an acousticreflector on the substrate, the acoustic reflector comprising aplurality of layers having alternating high and low acoustic impedances,outermost layers of the plurality of layers having the low impedance; apiezoelectric layer on the acoustic reflector; first and secondresonator electrodes on a same surface of the piezoelectric layer; and asensing layer contacting each of the first and second resonatorelectrodes, wherein the sensing layer covers only a portion of each ofthe first and second resonator electrodes.